1. Field of the Invention
The present invention relates mainly to a display apparatus having a matrix arrangement. Particularly, the present invention relates to a passive matrix display apparatus of a type that light transmittance is rapidly changed when a voltage applied to the intersection of a lateral electrode and a longitudinal electrode, i.e. a row electrode and a column electrode of a matrix exceeds a threshold value.
2. Discussion of the Background
A conventional technique will be described with reference to a liquid crystal display apparatus wherein a signal electrode and a scanning electrode are respectively referred to as a column electrode and a row electrode. In a passive matrix type liquid crystal display apparatus comprising an N.sub.r number of row electrodes and an M.sub.c number of column electrodes, a so-called successive line scanning method has been used to drive it. Namely, a group of pixel signals corresponding to pixels on an arbitrary line of row electrodes is applied to column electrodes, and at the same time, a column electrode selective voltage is applied to each row electrode. Thus, the light transmittance of each pixel selected by the application of the row electrode selective voltage is changed. The above-mentioned operation is effected to scan the N.sub.r number of row electrodes for each row electrode.
In the liquid crystal display apparatus, the light transmittance of a pixel has a threshold characteristic which relies on the effective value of a voltage applied to the pixel. In the above-mentioned driving method, it has been known that the condition to obtain the ratio of the maximum and minimum values of light transmittance, i.e. the largest contrast ratio can be expressed by formula (1) (reference document: Scanning Limitations of Liquid-Crystal-Displays, IEEE Transactions on Electron Devices, vol. ED-21, No. 2, February 1974, pp146-155 by Paul M. Alt, Peter Pleshko): EQU V.sub.r /V.sub.c =N.sub.r.sup.1/2 ( 1)
Under the condition of the formula (1), the ratio of the effective value V.sub.on of a pixel voltage which provides the maximum (or the minimum) light transmittance to the effective value V.sub.off of the pixel voltage which provides the minimum (or the maximum) transmittance is expressed by formula (2). EQU V.sub.on /V.sub.off =((N.sub.r.sup.1/2 -1)/(N.sub.r.sup.1/2 +1)).sup.1/2( 2 )
Further, V.sub.off is given by formula (3). EQU V.sub.off =V.sub.c (2(N.sub.r -N.sub.r.sup.1/2)/N.sub.r.sup.2).sup.1/2( 3)
From the formulas (1) and (3), formula (4) is obtained. ##EQU1##
The value V.sub.off is generally set to a threshold value V.sub.th of transmittance vs effective value characteristics. Accordingly, the values V.sub.c and V.sub.r are determined by the value of V.sub.th. Therefore, the conventional technique had a drawback that as the number of row electrodes was increased, a very higher value was required for the row voltage. In the passive matrix display apparatus, a gray shade display could be obtained by an amplitude modulation wherein a column voltage is changed depending on a degree of gray shade, or by changing a voltage application time in a case that a voltage applied to row electrodes is fixed to +V.sub.r or -V.sub.r in a selection time, and the voltage is 0.sub.v in a non-selection time. As a method for changing the application time, there are a method of changing the pulse width of a column voltage (pulse width modulation) and a method of changing the number of pulses while the pulse width is constant (pulse number modulation). To effect the pulse number modulation, for instance, one picture may be expressed by the number of frames (or the number of fields) corresponding to the number of gray shade levels, and column electrode pulses are applied to obtain V.sub.on depending on the gray shade of each of the pixels. Such method is called frame modulation.
The amplitude modulation, when it is done without any correction, results in non-uniformity of display because the effective value or the root mean square value of a voltage applied to a pixel in a non-selection time is variable depending on pixels. Further, the pulse width modulation may cause non-uniformity of display because a driving waveform distortion becomes large for a pixel remote from a driving point due to an electrode resistance when a signal having a narrow pulse width is applied. When the pulse width is sufficiently widened in the pulse width modulation, the frame frequency becomes too small so that a flicker of picture is resulted. Further, the frame modulation has a problem that a low frequency driving signal component increases and a flicker becomes conspicuous as the number of gray shade levels increase, unless the frame frequency is increased.